Dense deposit disease (DDD), also called membranoproliferative glomerulonephritis (MPGN) type II, is a rare form of glomerulonephritis named because of the characteristic appearance of electron-dense material in the glomerular basement membrane observed on kidney biopsy. The principle immune defect in DDD is excessive activation of the alternative complement pathway, with deposition of complement components in the glomerular basement membrane. Hence, by immunofluorescence microscopy, there is heavy C3 deposited along the basement membrane. Some patients have been found to have deficiencies of Factor H or Factor I, inhibitors of C3 activation. Others have a C3 nephritic factor, an antibody that activates the alternative complement cascade. It has recently been recognized that C3 nephropathy, a rare glomerular disease with mesangial cell proliferation and C3 deposition by immunofluorescence microscopy, is associated with similar over-activation of the alternative complete cascade.